1. Field of the Invention
The present invention relates to a discharge lamp lighting device capable of performing a high response control to light a discharge lamp using a transformer which detects a fluctuation of output in order to avoid any occurrence of acoustic resonance phenomenon which causes a trouble in Alternating Current (AC) lighting of the discharge lamp.
2. Description of the Related Art
FIG. 9 shows a circuit diagram of a discharge lamp lighting device using a high voltage pulse generator. In FIG. 9, reference number 1 designates a direct current (DC) power source, 2 denotes a DC/DC converter, 3 indicates a H-type bridge, 4 designates a high voltage pulse generator, and 5 indicates a High Intensity Discharge bulb (or a HID lamp).
In the DC/DC converter 2, reference number 6 designates a switching transistor, 7 indicates a transformer, 8 designates a diode, and 9 denotes a capacitor. Reference characters 10a-10d in the H-type bridge 3 designate switching transistors. In the high-voltage pulse generator 4, reference number 11 indicates a capacitance, 12 denotes a lighting switch, and 13 designates a lighting transformer.
Next, a description will now be given of the operation of the conventional discharge lamp lighting device.
In a starting of lighting for the HID bulb 5, the switching transistor 6 switches the voltage of the DC power source 1. The transformer 7 increases in voltage the obtained voltage and the diode 8 then rectifies the increased voltage. The obtained voltage of a relatively 400 Volts can thereby be supplied to the capacitance 9. The switching transistors 10a and 10d in the H-type bridge 3 enter ON in order to supply the DC voltage to both the electrodes of the HID bulb 5. In the high voltage pulse generation circuit 4, the lighting switch 12 enters ON and the lighting transformer 13 then increases the voltage accumulated in the capacitance 11. The obtained pulse voltage of approximately 20 KV is supplied to both the electrodes of the HID bulb 5. Thereby the HID bulb 5 initiates the discharge therein. After the completion of the lighting, the DC voltage to be supplied to the HID bulb 5 becomes from 400 Volts to 20 Volts, further, to approximately 85 Volts. After this, the switching transistors 10a-10d in the H-type bridge 3 enter ON in order to supply an alternating current (AC) voltage to the HID bulb 5, and the HID bulb 5 is thereby shifted to a steady lighting (or usual lighting).
A circuit configuration in which the diode 8, the capacitance 9, and the H-type bridge 3 are eliminated from the configuration shown in FIG. 9 shows a discharge lamp lighting device for AC lighting.
However, an acoustic resonance phenomenon frequently occurs in the discharge lamp lighting device for AC lighting. When the acoustic resonance phenomenon occurs in the discharge lamp lighting device, the arc discharge in the HID bulb 5 is fluctuated and the light emission in the HID bulb 5 become unstable.
Furthermore, when a large resonance occurs in the HID bulb 5, the acoustic resonance is reached to the inner wall of the HID bulb 5 and destroys the glass of the HID bulb 5.
In general, a resonance frequency is changed according to the shape of the glass bulb of the HID bulb 5 and the state of gases filled therein. Each HID bulb 5 have an inherent resonance frequency. A low frequency used in currently available lighting devices almost occurs no acoustic resonance phenomenon. In other words, the above low frequency is selected within a frequency range in which the stable lighting can be obtained. Therefore in order to light the HID bulb 5 out of the low frequency range for a stable lighting, it is necessary to avoid any occurrence of the acoustic resonance phenomenon. As has been well known, there are techniques to avoid an occurrence of the acoustic resonance phenomenon, for example, following Japanese patent documents 1 and 2.
Patent Document 1: Japanese laid open publication number S56-11895; and
Patent Document 2: Japanese laid open publication number H9-510046.
In order to avoid the occurrence of an acoustic resonance phenomenon, Japanese patent document 1 uses an acoustic resonance phenomenon detection means and a control means for changing a resonance frequency. This acoustic resonance phenomenon detection means detects a fluctuation of a light emitted in the HID bulb and a voltage of the electrode of the HID bulb. By the way, because the detection of the fluctuation of light requires a high accuracy detector, it is unrealistic to mount it in the discharge lamp lighting device in cost and size. In addition, only the detection of the voltage of the HID bulb cannot detect the acoustic resonance phenomenon preciously.
In order to avoid the occurrence of an acoustic resonance phenomenon, Japanese patent document 2 detects a voltage and a current of a HID bulb and the occurrence of an acoustic resonance phenomenon using a calculation means and a judgment means, and shifts a resonance frequency in order to avoid the occurrence of the acoustic resonance phenomenon. However, this technique must use a filter means before the calculation of the calculation means and the calculation result includes a time delay. Each phase is delayed in the filter means, and the calculation result thereby includes a time error. Accordingly, the circuit configuration becomes a complicated and it is difficult to detect the acoustic resonance phenomenon speedy and preciously.
Because the conventional discharge lamp lighting devices have the configurations described above, the technique of the patent document 1 including the means for avoiding the occurrence of an acoustic resonance phenomenon cannot detect the acoustic resonance phenomenon preciously, and the technique of the patent document 2 has a complicated configuration of the means for avoiding the occurrence of an acoustic resonance phenomenon, and cannot detect the occurrence of the acoustic resonance phenomenon speedy and preciously.
The present invention has been made to solve the above conventional drawback of the discharge lamp lighting device. It is therefore an object of the present invention to provide a discharge lamp lighting device with a simple configuration for speedy detecting and avoiding any occurrence of an acoustic resonance phenomenon.
In accordance with an aspect of the present invention, there is provided a discharge lamp lighting device having a fluctuation detection circuit for outputting a fluctuation signal corresponding to a change of an output power value obtained by adding detected values of a voltage detection means and a current detection means. A switching circuit is controlled in operation based on the fluctuation signal outputted.